DESCRIPTION: The multiple drug resistance (mdr) genes encode P- glycoproteins (Pgp) which function as versatile ATP dependent pumps, transporting a variety of structurally unrelated substances from the cytoplasm to reduce the intracellular accumulation of cytotoxic drugs. Pgps were initially identified in tumor cells but are now known to be expressed in normal cells including the lumenal surface of intestinal epithelia. Not all Pgps confer drug resistance. The principal investigator has shown in rat and man that only the functional forms, mdr1a and mdr1 respectively, are significantly expressed in small intestine. Preliminary data also suggests that mdr functions in the intestine as part of a barrier to xenobiotics. The principal investigator has also found that there is significant interindividual variation in the expression of mdr1 mRNA and protein in the small bowel. The principal investigator believes this heterogeneity contributes to interpatient differences in the oral clearance of mdr substrates and may represent a previously unrecognized risk factor for environmental diseases. The first hypothesis to be tested is that variation in expression of mdr1 in human intestine is largely due to non genetic factors. The principal investigator bases this hypothesis on his preliminary studies that demonstrated mdr1a mRNA and protein are highly inducible in rat intestine by a variety of xenobiotics. In Specific Aim 1, the principal investigator will identify inducers of mdr1 mRNA and protein in small bowel and colon by treating cultured human intestinal explants with a battery of suspected inducers. In Specific Aim 2, the inducers identified in Aim 1 will be tested in vivo by measuring mdr1 mRNA and protein in small and large bowel biopsies obtained endoscopically form healthy subjects after a 4 day inducer free diet and again after short term treatment with the inducer. Intestinal explants obtained from each subject will also be exposed to the same inducer in vitro. The principal investigator will also test the effects of diet on intestinal mdr expression by placing healthy volunteers on diets containing charcoal broiled beef. The level of mdr1 mRNA and protein will be determined in small and large bowel biopsies obtained at study entry, after a 4 day inducer free diet, and after 7 days on the test diet. The principal investigator will collaborate with Dr. Paul Watkins on similar studies involving grapefruit juice (flavonoids) and cruciferous vegetables (indoles). Specific Aim 3 will directly assess the hypothesis that the level of intestinal mdr influences the absorption of drugs in the intestine. The principal investigator will first examine the effect of inducing mdr expression on the threshold for cyclosporine (CSA) absorption in an isolated loop of rat intestine. Correlation between mdr protein levels and transport function will also be determined in rat intestinal brush border vesicles (collaboration with Richard Moseley). Second, the principal investigator will determine the contribution of interindividual heterogeneity in intestinal mdr expression to the oral kinetics of CSA, midazolam, and 17 alpha- ethinyl estradiol in healthy subjects (collaboration with Paul Watkins). In Specific Aim 4, the principal investigator will determine whether CaCo-2 cells are responsive to the mdr inducers identified in the Specific Aims 1 and 2. The discovery of a responsive human cell line would allow the principal investigator to characterize the genetic differences in mdr expression in the future. It is proposed that the data obtained from the planned studies will lead to novel and testable hypotheses regarding the bioavailability of drugs and environmental risk assessment.